1. Field of the Invention
The present invention relates to a method for manufacturing a thin-film capacitor for properly performing temperature compensation of the junction capacitance of a semiconductor device, a thin-film capacitor device having the thin-film capacitor manufactured by the method, and an electronic circuit having the thin-film capacitor device for performing temperature compensation of the electronic circuit.
2. Description of the Related Art
Thin-film capacitor devices generally have a substrate and a lower electrode, a dielectric layer, and an upper layer which are deposited on the substrate in that order. The thin-film capacitor devices also have a semiconductor substrate functioning as a lower electrode and have a dielectric layer and an upper layer which are deposited on the semiconductor substrate in that order in some cases.
The thin-film capacitor devices are required to have a large relative dielectric constant and Q factor and a temperature coefficient of capacitance of near 0 at a resonant frequency.
Hitherto, the following compounds are known as dielectrics having the above characteristics: BaOxe2x80x94TiO2 dielectrics containing Sm2O3, Gd2O3, Dy2O3, or Eu2O3. However, when manufacturing such dielectrics using conventional techniques, the relative dielectric constant is adjustable only in the range of 61 to 72 and the temperature coefficient of capacitance is adjustable only in the range of xe2x88x9224 to 31 ppm/xc2x0 C.
For the above background, research and development has been conducted. A dielectric ceramic having the following structure has been proposed: a multilayer including a first dielectric ceramic sheet having a positive temperature coefficient of capacitance at a resonant frequency and a second dielectric ceramic sheet having a negative temperature coefficient of capacitance at a resonant frequency.
According to this method, the multilayered dielectric ceramic is prepared as follows: a mixture having a desired composition is formed into a disk having a diameter of 16 mm and a thickness of 9 mm, the disk is fired at 1,260xc2x0 C. to 1,450xc2x0 C. for several hours, a first dielectric ceramic is then obtained; and another mixture having a different composition from the above composition is formed into another disk, the disk is fired, and a second dielectric ceramic is then obtained; each of the first and second dielectric ceramics is cut into a sheet having a thickness of 1 mm; and both the sheets are layered to complete the multilayered dielectric ceramic. The relative dielectric constant and the temperature coefficient of capacitance of the multilayered dielectric ceramic can be adjusted by using materials having different relative dielectric constants or using sheets which are made of the same material and have different thicknesses.
According to the above method, since the multilayered capacitors are manufactured by layering the fired first and second dielectric ceramics, each having a thickness of 1 mm, the miniaturization and a reduction in thickness are limited. For example, thin-film capacitors having a thickness of 1 mm or less cannot be manufactured.
When the dielectric ceramic sheets are laminated, an adhesion layer or an air layer which has a different dielectric constant exists between the sheets. Such a structure has a plurality of portions, each having a different dielectric constant, in the thickness direction of the layered sheets; hence there is a problem in that it is difficult to manufacture capacitors having a desired temperature coefficient of capacitance.
Furthermore, the sheet dielectric ceramic is polycrystalline and subsequently has a plurality of grain boundaries in the thickness direction; hence, reduction in the dielectric loss at a high frequency of 1 GHz or more is difficult.
In thin-film capacitors, the desired thickness of the second dielectric thin-film is inversely proportional to a ratio of the absolute value of the temperature coefficient of capacitance to the relative dielectric constant (hereinafter referred to as a ratio xcfx84/xcexa). That is, as the thickness of the dielectric thin-film increases, as the absolute value of the ratio xcfx84/xcexa decreases. The relationship is more significant at a high relative dielectric constant. It is subsequently difficult to manufacture thin-film capacitors having a high relative dielectric constant and a small thickness even if the above methods are improved.
In a thin-film capacitor having a first dielectric thin-film and a second dielectric thin-film which are layered, the thickness of each of the first and second dielectric thin-films is determined according to the relative dielectric constant and the temperature coefficient of capacitance. When a capacitor device having a sheet capacitance of (C/S) pF/mm2 includes a dielectric thin-film (controlled film: first dielectric thin-film: referred to as film C) having a relative dielectric constant xcexaC and a temperature coefficient of capacitance of 0 ppm/xc2x0 C., and includes another dielectric thin-film (second dielectric thin-film: referred to as film N) having a relative dielectric constant xcexaN and a temperature coefficient of capacitance of xcfx84N ppm/xc2x0 C., the following formulas (1) and (2) are obtained for the thickness tC of the first dielectric thin-film, the thickness tN of the second dielectric thin-film, and dielectric ∈0 constant of vacuum:                               t          N                =                                                            ϵ                0                            ⁢                              xe2x80x83                            ⁢              τ                                      (                              C                /                S                            )                                ⁢                      xe2x80x83                    ⁢                      1                          (                                                τ                  N                                                  κ                  N                                            )                                                          (        1        )                        and                      xe2x80x83                                          t          C                =                                                            ϵ                0                            ⁢                              xe2x80x83                            ⁢                              κ                C                                                    (                              C                /                S                            )                                -                                    t              N                        ⁢                          xe2x80x83                        ⁢                                                            κ                  C                                                  κ                  N                                            .                                                          (        2        )            
The above formulas (1) and (2) show that the thickness xcfx84N of the second dielectric thin-film (film N) is determined according to the ratio xcfx84N/xcexaN. Hitherto, in order to obtain a second dielectric thin-film having a smaller thickness, changing the ratio xcfx84N/xcexaN, that is, increasing the ratio xcfx84N/xcexaN by using different materials, is required. That is, developing a new dielectric material having a large value of the ratio xcfx84N/xcexaN is necessary.
Accordingly, it is an object of the present invention to provide a method for manufacturing a thin-film capacitor for properly performing temperature compensation. In the thin-film capacitor, the miniaturization and a reduction in the thickness and the weight can be achieved. It is another object of the present invention to provide a thin-film capacitor device having the thin-film capacitor manufactured by the method. It is another object of the present invention to provide an electronic device having the thin-film capacitor manufactured by the method. It is another object of the present invention to provide an electronic circuit having the thin-film capacitor device manufactured by the method.
In order to solve the above problems, in a method for manufacturing a thin-film capacitor having a desired sheet capacitance and a desired temperature coefficient of capacitance by depositing a first dielectric thin-film having a temperature coefficient of capacitance with an absolute value of 50 ppm/xc2x0 C. or less and a second dielectric thin-film having a negative temperature coefficient of capacitance, wherein the second dielectric thin-film has a structure composed of an aggregation of principal grain units each having a principal crystal grain and grain boundary layers surrounding the principal crystal grain, includes a plurality of principal grain units, and has a thickness tN, wherein tN={∈0xcfx84t0t/(C/S)}xc2x7{1/(xcfx84/xcexa)}, wherein C/S represents the sheet capacitance, ∈0xcfx84t0t represents the desired temperature coefficient of capacitance, xcfx84 represents the temperature coefficient of capacitance of the second dielectric thin-film, and xcexa represents the relative dielectric constant of the second dielectric thin-film, the method includes determining a target value of a grain size of the second dielectric thin-film by selecting the grain size satisfying the formula (xcfx84/xcexa)/(xcfx84g/xcexag) greater than 1, wherein xcfx84g represents the temperature coefficient of capacitance of the principal crystal grain, and xcexag represents the relative dielectric constant of the principal crystal grain, and depositing the second dielectric thin-film so that the grain size becomes the target value to reduce the thickness of the second dielectric thin-film.
According to the present invention, thinner thin-film capacitors in which a reduction in the thickness and the weight is achieved can be manufactured without using a newly developed material. For a thin-film capacitor having a first dielectric thin-film and a second dielectric thin-film which are laminated, the inventors have been discovered that a thin-film capacitor having a smaller thickness and a desired relative dielectric constant and temperature coefficient of capacitance can be obtained by adjusting the size of a principal crystal grain in the second dielectric thin-film. Based on the relationship, thin-film capacitors having a smaller thickness can be obtained. That is, thin-film capacitors having a thinner dielectric thin-film can be obtained by adjusting the size of the principal crystal grain so that the ratio (xcfx84/xcexa)/(xcfx84g/xcexag) exceeds 1, wherein the ratio (xcfx84/xcexa) belongs to the dielectric thin-film and the ratio (xcfx84g/xcexag) belongs to the principal crystal grain.
In order to solve the above problems, in a method for manufacturing a thin-film capacitor having a desired sheet capacitance and a desired temperature coefficient of capacitance by depositing a first dielectric thin-film having a temperature coefficient of capacitance with an absolute value of 50 ppm/xc2x0 C. or less and a second dielectric thin-film having a negative temperature coefficient of capacitance, wherein the second dielectric thin-film has a structure composed of an aggregation of principal grain units each having a principal crystal grain and grain boundary layers surrounding the principal crystal grain, includes a plurality of principal grain units, and has a thickness tN, wherein tN={∈0xcfx84t0t/(C/S)}xc2x7{1/(xcfx84/xcexa)}, wherein C/S represents the sheet capacitance, ∈0xcfx84t0t represents the desired temperature coefficient of capacitance, xcfx84 represents the temperature coefficient of capacitance of the second dielectric thin-film, and xcexa represents the relative dielectric constant of the second dielectric thin-film, the method includes determining a target value of a grain size of the second dielectric thin-film by selecting the ratio a/2xcex94a satisfying the formula (xcfx84/xcexa)/(xcfx84g/xcexag) greater than 1 when the ratio b/a is constant, wherein xcfx84g represents the temperature coefficient of capacitance of the principal crystal grain, xcexag represents the relative dielectric constant of the principal crystal grain, a represents the width of each principal grain unit, which includes the principal crystal grain and the grain boundary layers, in the lateral direction, xcex94a represents the thickness of each grain boundary layer, 2xcex94a represents the thickness of a grain boundary, and b represents the height of the principal grain unit, and depositing the second dielectric thin-film so that the grain size becomes the target value to reduce the thickness of the second dielectric thin-film.
According to the present invention, thinner thin-film capacitors in which a reduction in the thickness and the weight is achieved can be manufactured without using a newly developed material. For a thin-film capacitor having a first dielectric thin-film and a second dielectric thin-film which are laminated, the inventors have been discovered that a thin-film capacitor having a smaller thickness and a desired relative dielectric constant and temperature coefficient of capacitance can be obtained by adjusting the size of a principal crystal grain in the second dielectric thin-film using the relationship between the ratio a/2xcex94a of a principal crystal grain in the dielectric thin-film and the ratio xcfx84/xcexa of the dielectric thin-film. Based on the relationship, thin-film capacitors having a smaller thickness can be obtained.
That is, thin-film capacitors having a thinner dielectric thin-film can be obtained by adjusting the size of the principal crystal grain so that the ratio (xcfx84/xcexa)/(xcfx84g/xcexag) exceeds 1, wherein the ratio (xcfx84/xcexa) belongs to the dielectric thin-film and the ratio (xcfx84g/xcexag) belongs to the principal crystal grain.
In order to solve the above problems, in a method for manufacturing a thin-film capacitor having a desired sheet capacitance and a desired temperature coefficient of capacitance by depositing a first dielectric thin-film having a temperature coefficient of capacitance with an absolute value of 50 ppm/xc2x0 C. or less and a second dielectric thin-film having a negative temperature coefficient of capacitance, wherein the second dielectric thin-film has a structure composed of an aggregation of principal grain units each having a principal crystal grain and grain boundary layers surrounding the principal crystal grain, includes a plurality of principal grain units, and has a thickness tN, wherein tN={∈0xcfx84t0t/(C/S)}xc2x7{1/(xcfx84/xcexa)}, wherein C/S represents a sheet capacitance, ∈0xcfx84t0t represents a desired temperature coefficient of capacitance, xcfx84 represents the temperature coefficient of capacitance of the second dielectric thin-film, and xcexa represents the relative dielectric constant of the second dielectric thin-film, the a method includes depositing the second dielectric thin-film so that the ratio (xcfx84(x)/xcexa(x))/(xcfx84g/xcexag) is 1.10 or more, wherein xcexa(x)/xcexag=xcex3[(xxe2x88x921)2/(xcex3xxe2x88x921+xcexag/xcexagb)+{(2xe2x88x921/x)/xcex3}/(xcexag/xcexagb)]/x, that is, xcexa(x) indicates that the xcexa is the function of x, xcfx84(x)/xcfx84g=1xe2x88x92[(xcexag/xcexagb)xc2x7(1xe2x88x92xcfx84gb/xcfx84g)xc2x7{(xcexag/xcexagb)2(xxe2x88x921)2+(xcex3xxe2x88x921+xcexag/xcexagb)2(2xe2x88x921/x)xcex3}/(xcexag/xcexagb)xc2x7(xcex3xxe2x88x921+xcexag/xcexagb)xc2x7{(xxe2x88x921)2xc2x7(xcex3xxe2x88x921+xcexag/xcexagb)xc2x7(2xe2x88x921/x)/xcex3}], xcfx84g represents the temperature coefficient of capacitance of the principal crystal grain, xcexag represents the relative dielectric constant of the principal crystal grain, x, which is a dimensionless parameter, represents the ratio a/2xcex94a, xcex3, which is a dimensionless parameter, represents the ratio b/a, xcexagb represents the relative dielectric constant of the grain boundary, a represents the width of each principal grain unit, which includes the principal crystal grain and the grain boundary layers, in the lateral direction, xcex94a represents the thickness of each grain boundary layer, 2xcex94a represents the thickness of a grain boundary, and b represents the height of the principal grain unit.
According to the present invention, thinner thin-film capacitors in which a reduction in the thickness and the weight is achieved can be manufactured without using a newly developed material. For a thin-film capacitor having a first dielectric thin-film and a second dielectric thin-film which are laminated, the inventors have been discovered that a thin-film capacitor having a smaller thickness and a desired relative dielectric constant and temperature coefficient of capacitance can be obtained by adjusting the size of a principal crystal grain in the second dielectric thin-film using the relationship between the ratio a/2xcex94a of a principal crystal grain in the dielectric thin-film and the ratio xcfx84/xcexa of the dielectric thin-film. Based on the relationship, thin-film capacitors having a smaller thickness can be obtained.
In the present invention, the second dielectric thin-film is manufactured such that the ratio (xcfx84(x)/xcexa(x))/(xcfx84g/xcexag) is 1.25 or more.
When the ratio (xcfx84(x)/xcexa(x))/(xcfx84g/xcexag) is 1.25 or more, a further reduction in the thickness and the miniaturization can be achieved compared with a state in which the ratio (xcfx84(x)/xcexa(x))/(xcfx84g/xcexag) is 1.10.
In the present invention, the averages of the ratio a/2xcex94a and the ratio b/a satisfy the conditions 1.7xe2x89xa6a/2xcex94axe2x89xa613 and 5xe2x89xa6b/a.
In the present invention, the averages of the ratio a/2xcex94a and the ratio b/a satisfy the conditions 1.8xe2x89xa6a/2xcex94axe2x89xa66 and 5xe2x89xa6b/a.
When the above conditions are satisfied, a reduction in the thickness and the miniaturization can be achieved.
In the present invention, the second dielectric thin-film has a relative dielectric constant of 100 or more.
In the present invention, the second dielectric thin-film includes any one selected from the group consisting of SrxBa1xe2x88x92xTiO3, CaTiO3, and PbTiO3.
When the dielectric thin-film includes the material, adjusting the grain size is possible; thereby achieving a reduction in the thickness and the miniaturization by adjusting the grain size.
A thin-film capacitor device according to the present invention performs temperature compensation and includes electrodes and a thin-film capacitor placed therebetween, wherein the thin-film capacitor is manufactured by any one of the above methods.
An electronic device includes a thin-film capacitor manufactured by any one of the above methods.
The thin-film capacitor has a desired temperature coefficient of capacitance and a reduction in the thickness and the miniaturization can be achieved.
An electronic circuit according to the present invention includes a thin-film capacitor device and a varactor diode connected in parallel to the thin-film capacitor device, wherein the thin-film capacitor device has electrodes connected to input/output terminals and has a thin-film capacitor which is manufactured by any one of the above method and is placed between the electrodes.
Since the varactor diode has a positive temperature coefficient of capacitance, the temperature coefficient of capacitance of the varactor diode can be compensated by the temperature coefficient of capacitance of the thin-film capacitor device by connecting in parallel the varactor diode to the thin-film capacitor device; thereby achieving excellent temperature stability.